High efficiency laser cavity

ABSTRACT

A plurality of structural members form a laser cavity structure which has an inner cylindrical surface in which grooves have been formed for mounting light emitters. Other portions of the inner surface of the cavity are coated with a highly light reflective layer. Selected portions of the cavity wall contact the laser rod assembly which is positioned within the cavity and can be used as a heat sink for both the light emitter arrays and the laser rod assembly. Additional cooling for tubular laser rods is provided by a rod-like heat sink mounted concentric with the tubular rod.

United States Patent Scalise 1 Aug. 8, 1972 [54] HIGH EFFICIENCY LASERCAVITY 3,219,585 1 1/1965 Kaiser ..331/94.5 Inventor: S l y J. s DallasTex. Keck 6t 3] [73] Assignee: Texas Instruments Incorporated, primaryExamine, wi||iam Sikes Dallas Att0rneyHarold Levine, James 0. Dixon,Andrew [22] Filed; Oct 13 1970 M. Hassell, Melvin Sharp and Rene E.Grossman [21] Appl. No.: 80,423 57 ABSTRACT A plurality of structuralmembers form a laser cavity [52] US. Cl ..33l/94.5, 330/43 structurewhich has an inner Cylindrical Surface in o ..I'I01S grooves ha,e beened f n g [58] Field of Search ..33l/94.5; 330/4.3 emitters otherportions of the inner Surface of the cavity are coated with a highlylight reflective layer. [56] References cued Selected portions of thecavity wall contact the laser UNITED STATES PATENTS rod assembly whichis positioned within the cavity and can be used as a heat sink for boththe light emitter ar- 3,339,150 8/1967 Bowness "331/945 rays and thelaser rod assembly. Additional cooling for 3,413,567 11/1968 Hannwackeret al.....331/9 tubular laser rods is provided by a rod-like heat sink3,423,692 1/1969 Young ..33l/94.5 mounted concentric with the tubularrod 3,222,615 12/1965 Holly ...33l/94.5 3,102,920 9/1963 Sirons ..33l/94.5 7 Claims, 8 Drawing Figures PATENTED 3'97? 3.683.296

sum 1 or 3 nwmro/e Stan/ey J, Sea/1'59 ATTORNEY PATENTED M16 8 1912 saw3 or 3 Fig. 6

This invention relates to lasers and more particularly to solid-statelasers including laser cavity structures incorporating heat sinks.

At the present time a number of difl'erent lasers are known and in use.These include crystalline lasers, liquid lasers, glass lasers, gaslasers, and semiconductor or solid-state lasers. Laser cavity structuresincorporating heat sinks, which are the subject of this invention, areparticularly useful with crystalline lasers and other laser materialswhich can be formed into laser rods.

Many crystalline laser materials have relatively low coeflicients ofabsorption and the light output of solidstate light emitting diodearrays, which are the preferred light sources for use with solid-statelaser rods, is relatively low. Considering these factors, solidstatelasers must be highly efficient if a reasonable amount of output poweris to be obtained. Significant factors effecting the efliciency ofsolid-state lasers are the operating temperature of the laser rod andthe effective coupling coefficient of the light source to the laser rod.

The primary factors determining the temperature of the laser rod are thepower output of the light source, the ratio of absorbed to radiatedpower, the optical coupling coefficient between the light source and thelaser rod, and the means used to cool the rod. Choosing a laser materialessentially determines the ratio of absorbed to radiated power, thisfunction being an inherent characteristic of the laser material. Thecoupling coefficient is a function of the light source, the laser cavitystructure and the material used for the laser rod. In most cases thechoice of material for the laser rod is limited to relatively fewmaterials by the desired output spectrum. Light sources having arelatively high output and high optical coupling coemcients between thelight source and the laser rod are preferable even though these tend toincrease the temperature of the laser rod. This essentially limits thecontrol of the laser rod temperature to the use of heat sinks or someother means for removing heat from the laser rod.

The primary factors determining the optical coupling coefficient betweenthe light source and the laser rod are: the material used in the rod,the spectrum of the light source, and the design of the cavity in whichthe laser is mounted. Considerations other than coupling coefficient maydetermine the light source and the laser material to be used.

Considering the above discussed factors, improved heat sinks to moreeffectively cool the laser rod and improved cavity structure to increasethe coupling coefficient between the light source and the laser materialoffer the most flexibility in improving the efiiciency of solid-statelasers.

One embodiment of the invention provides a laser sections, structurecomprising a plurality of section, the sections being designed such thatwhen assembled they form a laser cavity which is substantiallycylindrical in shape with one or more grooves in the inner walls of thecavity for mounting light sources. A laser rod assembly is mounted inthe cavity and the interior surfaces of the cavity which are exposed tolight are coated with a highly light reflective layer such that anylight which passes through the laser rod assembly unabsorbed will bereflected through the rod assembly in a multi-path.

pattern. The laser rod may be surrounded by a cladding which istransparent to the light emitted by the light source with the outerdimensions of the cladding substantially conforming to the innerdimensions of the cavity. Alternatively, the outer dimensions of thelaser rod can be such that they substantially conform to the innerdimensions of the cavity. In either case the cavity can be used as aheat sink for the rod, the cladding if used, and the light sources. Theeffective coupling coeflicient between the light source and the laserrod is increased by the multi-path reflective characteristics of theinterior surfaces of the cavity.

In accordance with another embodiment of the invention, a laser cavitystructure for grooved laser rods is provided. This embodiment assuresthat substantially all the light emitted by a diode array light sourcewill pass through some portion of the laser rod before it impinges onany portion of the interior of the laser cavity structure. This isachieved by providing mounting surfaces on the inner wall of the cavitystructure for the light emitting diode arrays such that the lightemitting surfaces of the.diodes extend into grooves in the laser rod.

In accordance with another embodiment, a heat sink is provided for atubular laser rod assembly. The laser rod assembly may be mounted in thecavity structure discussed above. Additional cooling of the tubularlaser rod is provided by a second heat sink. The second heat sink has arod-like portion which is mounted concentric with the tubular laser rod.One end of the rod-like portion is secured to a base portion. Heat istransferred by conduction from the laser rod to the rod-like portion andalong the rod-like portion to the base. The base portion may be attachedto an additional heat dissipating structure to further augment cooling.Alternatively, the rod-like portion may consist of concentricallymounted tubes with an opening between the tubes thereby permitting acoolant to be circulating through the rod-like portion.

In each of the above discussed embodiments a laser cavity structure isprovided in which a laser rod assembly can be mounted. The cavitystructure may serve as a support and a heat sink for the laser rodassembly. The laser rod assembly can be a simple laser rod or it mayinclude other parts, a laser rod cladding for example.

FIG. 1 is an exploded view of a laser cavity structure including a laserrod assembly and light emitting diode arrays.

FIG. 2 is a cross section view of the assembled laser cavity structureof FIG. 1.

FIG. 3 is an isometric view of a grooved laser rod assembly and onesection of a cavity structure for use with this rod assembly.

FIG. 4 is a cross section view of the laser cavity structure illustratedin FIG. 3.

FIG. 5 is an exploded view of a tubular laser rod assembly including aheat sink.

FIG. 6 is an assembled tubular laser rod assembly with portions shown incross section.

FIG. 7 is an isometric view of a heat sink for tubular laser rodsconsisting of two concentrically mounted tubes through which a coolantis circulated.

FIG. 8 is an isometric view of an assembled laser cavity structureincluding cooling jacket through which a liquid or gaseous coolant maybe circulated.

Referring to the drawings and particularly to FIG. 1, a laser cavitystructure according to this invention is shown. The cavity structureconsists of three identical sections 10 which when assembled form asubstantially cylindrical cavity having grooves 12 in which lightemitters 14 are mounted. Positioned within the cavity is a laser rodassembly consisting of a laser rod cladding 16 mounted concentricallywith a laser rod 18. The light emitters 14 are a plurality of lightemitting diodes mounted in grooves in the inner wall of the cavity withthe diodes being oriented such that the emitted light passes into thecladding and tends to be focused by the cylindrical surfaces of thecladding 16 toward the center of the laser rod 18. This focusing of theemitted light toward the center of the rod 18 tends to cause the laseroutput to be concentrated near the longitudinal axis of the rod therebyincreasing the overall efficiency of the laser. The inner cylindricalwalls of the cavity are coated with a highly light reflective layer 20such that any light passing through the rod and cladding unabsorbed andstriking these surfaces tends to be reflected back into the rod 18 alonga multi-pass path. This multiple path geometry of the cavity inconjunction with the focusing effect of the cladding tends to greatlyincrease the coupling between the light emitters 14 and the laser rod18. The assembled cavity is shown in cross section in FIG. 2. Suitablemeans (not shown) must be provided for holding the cavity together andfor supplying power to the diodes. The cavity may be held together byscrews, for example, while power may be supplied to the diodes by leadspassing through openings in the cavity sections 10.

FIG. 3 illustrates one section 110 of a laser cavity structure whichminimizes the amount of light striking the interior walls of the cavityprior to passing through the laser rod assembly 118. In this embodimentthe laser rod assembly 118 has one or more grooves 117 formed along itslongitudinal surface 119 with the light emitters 112 being mounted onthe cavity section 110 such that when the cavity is assembled, the lightemitting surfaces of the diodes extend into these grooves 117 therebyassuring that substantially all of the emitted light will pass throughat least a portion of the laser rod assembly 118 before it impinges onany portion of the cavity wall. This is particularly advantageousbecause decreasing the amount of light which impinges on the cavitywalls before it passes through the laser rod assembly 118 increases theoptical coupling coefficient between the light emitters 112 and thelaser rod assembly 118. As with the above discussed embodiment, theouter surfaces of the laser rod assembly 118, with the exception of thegrooves, contact the inner surface of the cavity structure therebyassuring good thermal contact between the rod 118 and the cavitystructure. The assembled cavity structure including a laser rod assemblyis shown in cross section in FIG. 4. Suitable means (not shown) must beprovided for supplying power to the diodes and for holding the sections110 together. The cavity may be held together by screws, for example,while power may be provided to the diodes by leads passing throughopenings in the cavity sections 110.

The cavity structure illustrated in FIGS. 1 and 3 serve as a supportingstructure and as a heat sink for both the light source and the laser rodassembly. To increase the efiiciency of the cavity structure as a heatsink, the structural members should have large areas in direct contactwith the light emitters and the laser rod assembly or some alternatetechnique should be used to assure a low thermal resistance ismaintained between the light emitter, the laser rod assembly and thecavity structure. A useful technique for lowering the thermal resistancebetween the laser rod assembly and the cavity structure is to coat thelaser rod assembly with glycerol [C I-I (OI-I) Sufficient glycerolshould be used to assure no voids are left in areas where the laser rodassembly may not be in direct contact with the inner wall of the lasercavity structure.

FIG. 5 illustrates a laser rod assembly consisting of a laser rod 218and a heat sink 210. The laser rod 218 is provided with a bore throughits center thereby permitting the laser rod 218 to be positionedconcentrically with the heat sink 210. The outer dimension of the heatsink 210 is substantially the same as the inner dimension of the laserrod 218. One end of the heat sink 210 preferably includes a base and amounting stud as shown in FIG. 6. Heat is transferred from the laser rod218 to the heat sink 210 by conduction. Cooling can be further augmentedby attaching the heat sink 210 to a base (not shown) which serves tofurther increase the heat dissipation surface. Liquid or gaseouscoolants may be used to further increase the heat dissipation ratio ofthe base.

Another embodiment of a heat sink for use with a tubular laser rod isillustrated in FIG. 7. In this embodiment the heat sink 310 includes arod-like member which consists of two concentrically mounted tubes 312and 314. The inner tube is slightly shorter than the outer tube with theouter tube being closed at one end as shown generally at referencenumeral 316. Heat is transferred from the laser rod to outer tube 314 byconduction with the outer tube being cooled by a coolant flowing throughthe outer tube 314 and returning through the inner tube 312.

The performance of the above discussed heat sinks and the tubular laserrod can be further improved by coating the external surfaces of the heatsink and the rod-like portion of the heat sink with a highly lightreflecting layer and the inner surface of the tubular laser rod withglycerol before the laser rod is placed on the heat sink. The primarypurpose of the light reflecting layer is to reduce light absorption bythese surfaces. The primary purpose of the glycerol is to improve thethermal contact between the laser rod and the heat sink therebyincreasing the rate at which heat can be transferred from the rod to theheat sink by conduction. The performance of the laser illustrated inFIG. 1 can also be improved by coating the laser rod 18 with glycerolbefore it is placed in the cladding 16 for similar reasons.

Illustrated in FIG. 8 is a cylindrical laser cavity structure 410 and acooling jacket 420 mounted concentrically therewith. Space is providedbetween laser cavity structure 410 and the cooling jacket 420 such thatthe coolant is in direct contact with substantial portions of theoutside of the laser cavity structure. Two openings 422 and 424, onenear each end of the cooling jacket 420 are provided. Coolant isprovided by an external pump (not shown) with the coolant entering atthe first opening 422 and being discharged through a second opening 424.

The sections forming the cavity structures illustrated in FIGS. 2 and 4may be made of copper, aluminum, or other materials which have good heattransfer properties and sufficient rigidity to properly support thelaser rod assembly. The light emitters are preferably semiconductorlight emitting diodes with the exact diodes used depending somewhat onthe laser material used in that the spectrum of the emitted light shouldreasonably match the absorption spectrum of the laser material.Additionally the temperature at which the light emitters will beoperated must be considered as this can effect the spectrum of theemitted radiation. The choice of the laser material will dependprimarily on the desired spectrum for the emitted radiation. Neodymiumdoped yttrium aluminum garnet, commonly referred to as YAG has beenfound useful for lasers emitting radiation in the neighborhood of 10,600A. Diodes useful with the YAG rods include gallium arsenide phosphide,gallium phosphide, gallium aluminum arsenide and gallium arsenide.Liquid nitrogen has been found to be a suitable coolant for use with thecooling jacket illustrated in FIG. 8 or with the heat sink illustratedin FIG. 7. The light reflective layer for the inner surface of thecavity structure and exposed portions of the heat sink may be vacuumdeposited silver. Gold may also be used.

Although the invention has been disclosed and defined with reference topreferred embodiments, it will be obvious to those skilled in the artherein encompassed that many modifications are possible within theordinary skill of such artisans without departing from the scope of theinvention as herein disclosed and described.

WHAT IS CLAIMED IS:

1. A laser rod assembly comprising a tubular laser rod havingcylindrical inner and outer surfaces and a heat sink, said heat sinkhaving a rod-like surface concentrically positioned with respect to andin low thermal resistance contact with said inner cylindrical surface ofsaid laser rod and a base portion, said base portion including means forsecuring said base portion to a mounting structure such that heat isreadily transferred by conduction from said base portion to saidmounting structure.

2. A laser rod assembly including a tubular laser rod and a heat sink,said heat sink comprising first and second concentrically positionedtubular portions, said second tubular portion having an outer radiusless than the inner radius of said first tubular portion, said firsttubular portion being positioned concentrically with respect to saidlaser rod, the outer surface of said first tubular portion being in lowthermal resistance contact with the inner surface of said tubular laserrod, and means for circulating a coolant through said first and secondtubular portions thereby cooling said laser rod.

3. A laser structure comprising, a cylindrical laser rod assembly havingat least one groove along its outer surface, at least one member havingan inner surface shaped to substantially conform to the non-groovedportion of the outer surface of said laser rod assembly, 333 i523?- liis nhi fsii fiiie l m lr il 2 groove for mounting a light source suchthat the light emitting surface of said light source will be positionedwithin said groove in said laser rod assembly, and an array of solidstate light emitters mounted in said groove.

4. A laser structure in accordance with claim 3 wherein the innersurface of said at least one member is light reflecting.

5. A laser structure in accordance with claim 3 wherein said at leastone member is in low thermal resistance contact with said laser rodassembly and serves as a heat sink for cooling said laser rod assembly.

6. A laser structure in accordance with claim 3 wherein said laser rodassembly includes a laser rod and a cladding mounted concentric withsaid laser rod.

7. A laser structure in accordance with claim 3 wherein said solid statelight emitters are an array of semiconductor light emitting diodes.

1. A laser rod assembly comprising a tubular laser rod havingcylindrical inner and outer surfaces and a heat sink, said heat sinkhaving a rod-like surface concentrically positioned with respect to andin low thermal resistance contact with said inner cylindrical surface ofsaid laser rod and a base portion, said base portion including means forsecuring said base portion to a mounting structure such that heat isreadily transferred by conduction from said base portion to saidmounting structure.
 2. A laser rod assembly including a tubular laserrod and a heat sink, said heat sink comprising first and secondconcentrically positioned tubular portions, said second tubular portionhaving an outer radius less than the inner radius of said first tubularportion, said first tubular portion being positioned concentrically withrespect to said laser rod, the outer surface of said first tubularportion being in low thermal resistance contact with the inner surfaceof said tubular laser rod, and means for circulating a coolant throughsaid first and second tubular portions thereby cooling said laser rod.3. A laser structure comprising, a cylindrical laser rod assembly havingat least one groove along its outer surface, at least one member havingan inner surface shaped to substantially conform to the non-groovedportion of the outer surface of said laser rod assembly, said innersurface being in contact with and supporting said laser rod assembly,said one member having a groove for mounting a light source such thatthe light emitting surface of said light source will be positionedwithin said groove in said laser rod assembly, and an array of solidstate light emitters mounted in said groove.
 4. A laser structure inaccordance with claim 3 wherein the inner surface of said at least onemember is light reflecting.
 5. A laser structure in accordance withclaim 3 wherein said at least one member is in low thermal resistancecontact with said laser rod assembly and serves as a heat sink forcooling said laser rod assembly.
 6. A laser structure in accordance withclaim 3 wherein said laser rod assembly includes a laser rod and acladding mounted concentric with said laser rod.
 7. A laser structure inaccordance with claim 3 wherein said solid state light emitters are anarray of semiconductor light emitting diodes.